Messi and Ronaldo can do things with their dominant foot that most footballers can't dream of. Despite that, they both can shoot with their weaker foot when required and have scored plenty of goals with their weaker foot to prove it. Similarly players like Henry, van Persie, Ronaldinho are adept at using both feet and - particularly in van Persie's case - it clearly made him a better player since he was as one footed as they come when he first joined Arsenal.
You're right in that players will (and should) default to their strongest foot when taking a free kick or a corner but in the middle of the game there are plenty of instances when trying to bring the ball back on your stronger foot costs your an opportunity. It's not a pre-requisite to becoming world class but it undoubtedly makes you a better player.
Having said that, I'm not too fussed if Bellerin is one footed. It doesn't add as much value for a fullback to use his weaker foot.
